In many devices which use batteries, there often arises the need to have an auxiliary power source to back up the main battery. An example of such an application are children's toys which use alkaline batteries to charge nickel cadmium batteries functioning as the main power source. Other applications include dual chemistry systems in which rechargeable batteries recharge other rechargeable batteries.
Problems arise however in combining different battery types in order to provide an auxiliary power source. Although hybrid battery systems often exhibit the combined advantages of the different battery types, the same systems can also exhibit the combined disadvantages of the different battery types. Secondly, in many applications the main power source may become over-discharged and cannot be immediately recharged by the backup power source. The configuration of many hybrid battery systems are not useful in those applications.
Other problems also exist for products relying on rechargeable nickel cadmium batteries for operating and/or backup energy. Such products often require a trade-off between charger cost, battery life and availability of stored energy. To minimize the exposure inherent in periods in which the nickel cadmium battery is in a discharged or semi-discharged state, a high charge rate is needed. The high charge rate, which prevents a shortened nickel cadmium battery life, is usually provided by a complex and costly multi-rate charger. Secondly, in applications in which the discharge time may extend beyond the time for which sized, multi-cell nickel cadmium batteries may be drawn down to a point at which a cell is driven in reverse, thus, shortening the battery life. Finally, rechargeable nickel cadmium batteries have required a higher charging voltage than the source inherently available in the device into which they are designed. For example, a 3.6 volt nickel cadmium battery needs more than 5.0 volts if it is to be relably charged with systems now existing in the art.